The present invention resides in a method of producing ductile iron.
In order to achieve the desired mechanical properties in iron castings, the liquid iron must have the correct composition and it must also contain suitable nuclei to induce the correct graphite morphology on solidification. The liquid iron must have a suitable ‘graphitisation potential’. This is determined mainly by its “carbon equivalent value”. It is normal practice to adjust the graphitisation potential by nucleation, e.g. by the controlled addition of so-called inoculants. Inoculants are mostly based on graphite, ferrosilicon or calcium silicide, with the ferrosilicon being the most commonly used.
Ductile iron, also known as spheroidal graphite (SG) iron or nodular iron differs from grey cast iron in that in the former, precipitation of graphite is in the form of discrete nodules instead of interconnected flakes. Promotion of precipitation of graphite into nodules is achieved by treating the liquid iron with a so-called nodulariser, commonly magnesium, prior to casting (and prior to inoculation). The magnesium may be added as pure metal, or more commonly as an alloy such as magnesium-ferrosilicon or nickel-magnesium. Other materials include briquettes such as “NODULANT” (TM), formed from granular mixtures of iron and magnesium, and hollow mild steel wire filled with magnesium and other materials. In general, the magnesium treatment should result in about 0.04% of residual magnesium in the liquid iron. There are however, a number of difficulties with this magnesium addition. Magnesium boils at a relatively low temperature compared to the liquid iron so there is a violent reaction due to the high vapour pressure of magnesium at the treatment temperature causing violent agitation of the liquid iron and considerable loss of magnesium in vapour form. In addition, during the treatment, oxide and sulphides are formed in the iron resulting in dross formation on the metal surface. This dross must be removed as completely as possible before casting. Also, residual magnesium in the liquid iron after treatment oxidises continuously at the metal surface where exposed to air, causing loss of magnesium which may affect the structure of the graphite spheroids, and the dross formed may result in harmful inclusions in the castings. The loss of magnesium to the atmosphere and in the formation of sulphides and oxides is variable and makes it difficult to predict the appropriate level of addition for a particular batch and also requires that the iron is ‘overdosed’ by as much as 100% or even more (50% or more of the magnesium may be lost). These factors are clearly disadvantageous in terms of cost, ease of handling and predictability in the mechanical properties and overall quality of the final castings.
Furthermore, magnesium is in fact a carbide promoter, so the level of inoculants required after magnesium treatment is relatively high. Since any scrap is generally returned to the beginning of the process for economic reasons, there is a tendency for the silicon content in the iron (derived from the inoculant and nodulariser additions) to rise over a period of time, limiting the proportion of scrap that can be used (the level of silicon required at the end of the process is predetermined by the specification for the casting). Attempts have been made to mitigate the issues involved with magnesium addition. For example, Foseco have combined the addition of magnesium nodulariser with an addition of a barium alloy (e.g. that sold under the tradename “INOCULIN 390” and having the following composition (by weight %) 60-67Si, 7-11Ba, 0.8-1.5Al, 0.4-1.7Ca, the balance being Fe). All compositions presented hereinafter are presented as weight % unless indicated otherwise. The use of such alloys can mitigate some of the issues outlined above but not in a reliable and predictable manner.